<html>
<head>
</head>
<?php
$pollVals = array_map('intval', array($_POST['n1'], $_POST['n2'], $_POST['n3']) );
?>
<body>
<form method="post">
<input type="text" name="n1"/>
<input type="text" name="n2"/>
<input type="text" name="n3"/>
Максимальное значение : <?php echo max($pollVals); ?>
<br /><input type="submit" name="ok" value="OK">
</form>
</body>
</html>